Today, sensor data from multiple sensor devices is widely used. Various sensor devices such as gyroscopes and magnetometers are actually used in high-performance mobile equipment such as smart phones. Moreover, research is made on techniques such as a dead-reckoning (DR) technique in which multiple pieces of sensor data are integrated to perform position estimation, which is applicable to navigation systems. Thus, there is a need for techniques for integrally processing sensor data from multiple sensor devices.
In integral processing of multiple sensor devices as described above, in order to operate the sensor devices at low power consumption without interfering with the use of the user, it is important to reduce the load on an integral-processing-unit which obtains and processes the sensor data and to avoid unnecessary or inefficient measurements by performing optimal measurement management for each of the sensor devices.
For example, Patent Literature 1 discloses a sensor device which autonomously operates independent of an integrated processing-unit to reduce the load on the integrated-processing-unit.
FIG. 1 is a configuration diagram of a conventional GPS receiver with a DR sensor sampling function. The GPS receiver of FIG. 1 is the GPS receiver described in Patent Literature 1. This GPS receiver has a GPS positioning function as well as other functions required as a car navigation system. This GPS receiver can also provide information to a CPU (central processing unit) in a form easy to use. The GPS receiver in FIG. 1 includes: a GPS antenna 8; the CPU 10; a gyroscope 12; filters 14 and 16; the GPS receiver 20; a controlling-unit 22; an A/D converter 24; a counter 26; and a navigation system 100.
The GPS receiver described in Patent Literature 1 includes: sampling means for sampling DR sensor signals such as a gyroscope, a vehicle speed pulse, and a reverse signal at predetermined cycles; and controlling means for outputting multiple pieces of sampling data sampled by the sampling means together with a GPS position calculated at predetermined cycles, as one frame of data, every cycle in which the GPS position is calculated. In the case where the controlling means outputs one frame of data as serial data, the CPU only controls a serial interface to obtain the sampling data of the DR sensors together with the GPS position.
In other words, in the GPS receiver described in Patent Literature 1, the sensor devices independent of the CPU manage the data measurements by the sensor devices and transmit data in one set to the CPU to reduce the processing of the integrated-processing-unit which has been conventionally implemented by the CPU. Moreover, the GPS receiver performing measurements at a first measurement rate (low) has the controlling-unit which manages the data measurements by the devices for DR at a second measurement rate (high), and transmits data in one set to the CPU at the first measurement rate.
In a system in Patent Literature 1, instead of the CPU, a device to be a main device controls and manages the data measurements by sensor devices. Accordingly, the sensor devices can perform data measurements while being synchronized at a desired measurement rate (the number of measurements within a particular period of time), without increasing the load on the CPU. However, in the technique in Patent Literature 1, the functions of the CPU are merely alternatively implemented by the main device and a specific device with functions of controlling and managing data measurements is necessary. Thus, the sensor devices connected to the main device cannot autonomously operate.
There are, as sensor devices capable of full autonomous operation, sensor devices which have independent clock and performs autonomous measurements. Such sensor devices can thereby perform autonomous measurements at respectively set measurement rate, without putting a load on the CPU. However, such sensor devices have a problem that, since such sensor devices operate by independent clocks, measurement timing (output time at which measurement data is output to the CPU) deviates from one such sensor device to another, and hence the sensor devices cannot synchronize each other.
In view of this, there are recently another type of sensor devices which performs autonomous measurements synchronized with a measurement timing by receiving a trigger from outside the device.
For example, a magnetic sensor described in Patent Literature 2 is a magnetic sensor capable of performing synchronized samplings among each detecting-units (sensors) by detecting a trigger from the outside.
FIG. 2 is a block diagram for explaining the magnetic sensor described in Patent Literature 2. A magnetic sensor 9 in FIG. 2 corresponds to magnetic sensors 9a and 9b. The magnetic sensor 9 includes multiple magnetism-detecting-units 11a to 11c, sampling-process-units 17a to 17c, a trigger-detecting-unit 13, and a hold-process-unit 19. Detection signals from the multiple magnetism-detecting-units 11a to 11c are subjected to sampling processes by the sampling-process-units 17a to 17c, respectively. Moreover, the signals subjected to the sampling processes by the sampling-process-units 17a to 17c are inputted to the hold-process-unit 19, held until the next signals are inputted, and appropriately outputted to the outside. Moreover, the sampling-process-units 17a to 17c are driven in synchronization with an external trigger signal inputted to the trigger-detecting-unit 13 to perform sampling only once. Moreover, drive circuits of the magnetic sensors 11a to 11b can be also driven in synchronization with the external trigger signal inputted to the trigger-detecting-unit 13. Note that the magnetic sensor 9 includes a controlling-unit 15.
Such a technique allows multiple sensor devices to perform measurements synchronized with a measurement timing, without putting a load on CPU. There is a technique in which multiple sensor devices having the same measurement rate perform synchronized measurements among the sensor devices with a measurement timing as in aforementioned Patent Literature 2. However, in reality, different measurement rates and/or measurement timings are frequently required among different types of sensor devices. Measurement rates and/or measurement timings often vary among different types of sensor devices for power saving, for optimization, or due to difference in time required for measurements. Thus the measurement timings of the sensor devices can easily deviate from each other. Thus, in a case where multiple sensor devices are to be integrally processed, a correction process needs to be performed to correct deviations in measurement timing and associate each data sample. Patent Literature 3 can be given as an example. In Patent Literature 3, process timing of sensor devices are matched each other by setting respective delay time for each sensor according to their deviations in measurement timing.
Generally, in a vehicle navigation device, an optimal current position is estimated by combining a position calculated by a dead reckoning method and a position calculated by a GPS (Global Positioning System). In the dead reckoning method, a current position is calculated by updating a previous measured position on the basis of a speed pulse indicating the speed of the vehicle and a turn angle velocity of the vehicle measured by a gyroscope, i.e. an angular velocity sensor. In a navigation system employing such a method, the position of the vehicle can be derived by the dead reckoning method even in a tunnel, an underground parking lot, or an area between high buildings, where reception of radio waves from GPS satellites is difficult. However, this is only possible under the assumption that the moving distance from the speed pulse and the angular velocity from the gyroscope, i.e. the bearing are correctly obtained respectively.
The navigation device described in aforementioned Patent Literature 3 is related to a navigation system configured to derive the angular velocity of a moving object on the basis of an output value of an gyroscope attached in a moving object and a function for conversion from the output value of the gyroscope to the angular velocity. The navigation device includes a measuring-unit configured to periodically measure measurement data including at least the bearing of the moving direction and the moving speed, on the basis of signals received from satellites, and adjusts an output timing of an output value of the gyroscope in such a way that the output timing matches the measurement timing of the measurement data in the measuring-unit. The navigation device described above is a device which, in this case, adjusts the output timing of the output value of the gyroscope according to an error between the detection timing of the output value of the gyroscope and the measurement timing of the measurement data and thereby reduces the error between the detection timing and the measurement timing.
However, in the navigation device in Patent Literature 3, there is a need to calculate the delay time for correcting the deviation in measurement timings in advance through experiments and set the delay time. Thus, this method has a problem that it is not versatile.